After merging D3383 we still have work to do:
- Group operators in the 3D viewport and object properties need to be updated to clarify the UI.
- Outliner drag & drop should be unified so dragging from the text and icon does not give a different behavior. Also dragging between outliners can work
- Full scene copy and delete scene don't copy and delete collection datablocks as expected currently.
- Unit tests update to new code.
- Static overrides of collections don't work well yet, and is not clearly communicated in the UI.
- Currently all collections must have a globally unique name, we intend to relax this requirement in some way.
- Performance can be improved to avoid layer collection syncing for unaffected scenes.